The present invention relates to a new organic electroluminescence device; a thin-film, light-weight and high definition organic electroluminescence device; new photoelectron devices using said devices, such as a thin-film flat panel display, small sized portable projection display, cellular phone display device, portable PC display, real-time electronic bulletin board, light emitting diode, laser, two-dimensional optical pattern generating device, optical computer, optical cross connector and optical router; as well as to the systems and services using them.
there has been a growing demand for a light-weight, high definition and less costly small-sized flat panel display for use in the various types of cellular phones, mobile terminals, mobile computers and car navigation systems being developed. For household and office use, a space saving desktop display, a flat panel display and wall-mounted TV sets are taking the place of conventional CRT tube displays. Especially, digital signal transmission on the order of hundreds to several gigabits/sec. has been put into commercial use in both wired and wireless methods, as a result of the increased use of the high-speed Internet and the progress of digital broadcasting. Time is shifting into an age where general users will exchange a huge amount of information on a real-time basis. Under these circumstances, flat panel displays are required to provide a higher speed display to permit digital processing, in addition to being still more light-weight, and having a higher definition, a higher luminance and a lower price.
The Liquid Crystal Display (LCD), Plasma Display (PD) and Field Emission Display (FED) are currently under study to meet these requirements. In addition to these flat panel displays, new types of flat panel display, referred to as Organic Electroluminescence Devices (OELD) or Organic Light Emitted Diodes (OLED), have begun to draw attention in recent years.
The organic electroluminescence device provides a method of causing fluorescent or phosphorescent organic molecules to emit light by allowing an electric current to flow to the organic compound sandwiched between a cathode and an anode, thereby displaying information. According to the References (“Major Issues of Organic LED Elements to be Solved and Practical Statistics” edited by the Organic Electronics Material Research Organization, Bunshin Publishing Co., mid-1999, P.1–11, and “Preface to Current Situation and Issues of Materials and Devices” by Yoshiharu SATO), organic electroluminescence devices have long been studied mainly with respect to semiconducting crystals, such as anthracene and perylene.
In 1987, Tang et. al. proposed a two-layered organic electroluminescence device laminated with a light emitting organic compound thin film and a hole transporting organic compound thin film (C. W. Tang and S. A. Van Slyke, Appl. Phys. Lett. 51, 913 in 1987). The starting point is that a dramatic improvement of light emitting characteristics is enabled (light emitting efficiency: 1.51 m/W, drive voltage; 10V and luminance: 1000 cd/m2). Since then, a pigment doping technique and high molecular OLED, low working function electrode, mask vacuum evaporation system, etc. have been studied.
In 1997, an organic electroluminescence device based on an electrical charge injection method, called a simple matrix system was partly put into commercial use. Further, a new organic electroluminescence device based on the electrical charge injection method, called an active matrix system is currently under study for development. Such an organic electroluminescence device is operated according to the following principle: A fluorescent or phosphorescent organic light emitting material is made into a thin film between a pair of electrodes, and electrons and holes are injected from positive and negative electrodes. In the organic light emitting material, the injected electron becomes an organic one-electron molecule (simply called an electron) entering the Lowest Unoccupied Molecular Orbital (LUMO) of a light emitting molecule. The injected hole becomes an organic one-hole molecule (simply called hole) entering the Highest Occupied Molecular Orbital (HOMO) of the light emitting molecule. In the organic material, they move toward the opposite electrode. In the middle of the movement, when an electron meets a hole, a singlet or triplet state of excitation of the light emitting molecule is formed. As it deactivates while radiating light, light is released.
Generally, many of the organic light emitting materials are those having a high quantum efficiency with respect to photoexcitation, as in the case of various laser pigments. If these materials are made to emit light by electrical charge injection, the electron and hole have a lower electrical charge transport performance since many organic compounds are insulators. A high voltage on the order of hundreds of volts was required in the initial organic electroluminescence device. However, using excellent electrical charge transporting performances of the organic electrophotographic photoconductor used as a photoconductor of a copying machine, a thin film is divided into two types according to function. One is the film used to transport an electrical charge (hole), and the other is the film used to emit light. This separation of functions of the thin films has improved the light emitting characteristics in the above-mentioned Tang's two-layered organic electroluminescence device.
Recently, a 3-layered organic electroluminescence device has been reported wherein the electron transport performance of another electrical charge is assigned to a third organic thin film. In addition, separated function type and multi-layered film type organic electroluminescence devices have been proposed, wherein thin films assigned to perform various functions are added; for example, an electrical charge injection layer is provided to improve the characteristics of injecting the hole and electron into the organic material and a hole stop layer to improve the probability of re-combination between the two. However, the basis for light emitting is light radiation in the process of deactivation in the state of excitation from the organic light emitting molecule contained in the organic light emitting layer. This basis remains unchanged.
According to the References (“Major Issues of Organic LED Elements to be Solved and Practical Statistics” edited by the Organic Electronics Material Research Organization, Bunshin Publishing Co., mid-1999, P.25–38, and Yuuji HAMADA, “Chapter 2. Current situation and issues of Light Emitting Material”), a great number of the fluorescent or phosphorescent organic light emitting materials are known to have been developed for a variety of purposes, such as ink, dye and scintillator materials. The organic electroluminescence devices are made of these organic light emitting materials. They can be broadly classified in terms of molecular weight into low molecular and high molecular types.
The low molecular type is formed into thin films according to a dry process, such as a vacuum evaporation method, while the high molecular type is formed into thin films according to the cast method. Failure in the formation of organic thin films is said to be one of the reasons why a highly efficient device could not be obtained as an organic electroluminescence device in earlier days before Tang. Conditions required especially for the low molecule type are as follows: (1) Production of a thin film (100 nm level) in the vacuum evaporation system, (2) maintainability of a uniform thin film structure after formation of the film (without segregation crystal), (3) fluorescent light quantum yield in the solid status, (4) appropriate carrier transport performance, (5) heat resistance, (6) easy refining, and (7) electrochemical stability, etc. Further, this type can be classified into two types according to the light emitting process, that is, the light emitting material where light is emitted by direct re-combination between electron and hole, and fluorescent material (or dopant material) where light is emitted by photoexcitation caused by the light emitting material. In addition, when viewed from the differences in chemical structure, the following materials are known; metallic complex type light emitting material (8-quinolinol, benzooxazol, azomethine, flavone, etc. as ligand, and Al, Be, Zn, Ga, Eu, Pt, etc. as central metal) and fluorescent pigment based light emitting material (oxadiazole, pyrazoline, distyryl arylene, cyclopentadiene, tetraphenyl butadiene, bisstyryl anthoracene, perylene, phenanthrene, oligothiophene, pyrazoloquinoline, thiadiazopyridine, laminated perovskite, p-sexiphenyl, spiro compound, etc.).
As described above, a great variety of materials and techniques have been studied on the light emitting material and device production process of the organic electroluminescence device. However, these studies have not yet completely clarified the efficiency where the amount of light can be emitted from such an organic electroluminescence device. According to the References (“Major Issues of Organic LED Elements to be Solved and Practical Statistics” edited by the Organic Electronics Material Research Organization, Bunshin Publishing Co., mid-1999, P.105–118, and “Chapter 1 Interpretation and Limit of Light Emitting Efficiency” by Tetsuo IZUTSU), optical energy taken out of the organic electroluminescence device is given in terms of the number of photons released for each of electrons or holes running through the device. If this is expressed in terms of external quantum efficiency of electroluminescence η100 (ext), the following relationship is known to hold:ηφ(ext)=ηext×ηφ(int)=ηext×[γ×ηr×ηf]  (1)where ηφ(int) is an internal quantum efficiency representing the number of photons released for each of the electrons or holes running through the device inside the device, and ηφ(ent) denotes the efficiency of discharging, out of the device, the light produced inside the device after having been reduced by reflection or absorption on the device boundary. γ shows the charge balance equivalent to the ratio of the numbers of the electrons and holes injected inside the device, and ηr indicates the singlet exciton generation efficiency denoting the ratio of emitting the i-term exciton contributing to light emitted from the injected electric charge. ηf denotes light emitting quantum efficiency representing the ratio of emitting light and deactivating in the singlet exciton.
The external quantum efficiency ηφ(ext) equivalent to the amount of light emitted out of the device can be broadly classified into three, that is, ηr and ηf determined by the properties of the light emitting material itself, γ determined by the ratio of injecting the electrons and holes into the device, and η(ext) determined by the device structure. ηr and ηf are efficiencies related to the physical properties of the light emitting material itself and are uniquely determined by the light emitting material. γ is the amount determined by the electrical potential difference between the electrode and organic layer adjacent thereto, the boundary potential and the ease of movement of the electrons and holes in the organic layer. It is an efficiency uniquely determined by the physical properties of the electrode material and device internal organic material. Of these factors, the charge balance γ≦1. The singlet exciton generation efficiency ηr is said to be the electrical charge spin ηr≦0.25. Light emitting quantum efficiency ηr<1 except in the super-radioactive process. Therefore, the portion of the factor determined by the organic material inside the device and electrode material (the portion [γ×ηr×ηf] in Formula (1)) is said to be 0.25 or less. On the other hand, according to the Reference (Greenham, R. H. Friend, D. D. C. Bradley. Adv. Mater. 6, 491 in 1994), the discharge efficiency is determined by the reflection and refraction of classical optics. Assuming that the refractive index of the light emitting layer is “n”, it is given by the following equation:ηext=1/(2n2)  (2)
The refractive index of the light emitting layer of many organic electroluminescence devices or the glass substrate holding them is about 1.6. Thus, ηext=0.2. From the above discussion, the external quantum efficiency of the external electroluminescence is ηφ(ext)≦0.2×0.25=0.05, and the external quantum efficiency is said to be 5% at most.
To put the organic electroluminescence device to commercial use, it is essential to improve the external quantum efficiency. The external quantum efficiency of the above-mentioned conventional organic electroluminescence device has an upper limit, so the development of an organic electroluminescence device having different functions is currently under way. One of the methods is to improve the light emitting quantum efficiency singlet exciton generation efficiency fir of the light emitting material itself. In the conventional charge injection and re-combination process, singlet exciton occurs at the ratio of 0.25, and triplet exciton occurs at the ratio of 0.75. By contrast, the triplet exciton is converted to a singlet exciton by spinning in a reverse direction through an inter-item intersection resulting from the spin/orbital angular moment interaction of the organic light emitting material containing heavy metal, or the triplet exciton that has occurred is converted into a singlet exciton through mutual collision of triplet excitons enclosed in the nano-level range, thereby increasing the ratio of the exciton making contribution to light emission. As the material having such a new exciton generation mechanism, an organic electroluminescence device capable of high-efficiency light mission through the use of factris (2-phenylpyridine) iridium [Ir(ppy) 3] is introduced in Reference (M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, and S. R. Forrest, Appl. Phys. Lett. 75, 4–6 in 1999).
Another method is intended to improve the external quantum efficiency outside the device by improving the discharge efficiency ηext. Namely, a uniform thin film structure without crystal segregation has been considered essential for the production of the organic electroluminescence device. In this case, the organic light emitting material constituting the light emitting layer is random-oriented in terms of space. So light has been emitted isotropically in all directions inside the device. By contrast, a means of controlling the light emitted in the direction parallel to the light emitting surface of the device and increasing the light emitted in the vertical direction is described in the Reference (Japanese Patent Laid-Open 40413/1992) which discloses an organic electroluminescence device having a light emitting layer comprising molecules uniaxially oriented, for example, by the rubbing method.
According to the Reference (Japanese Patent Laid-Open No. 102783-1999), in the organic electroluminescence device produced by forming a light emitting layer in the dry process in a vacuum and by orienting the organic molecules constituting the light emitting layer parallel to the light emitted surface by photoisomerization reaction, an anisotropic light emitting characteristic was similarly obtained inside the light emitting layer. However, improvement of the discharge efficiency by orientation is not specifically described in these References. Only the Reference (Japanese Patent Laid-Open NO. 102783/1999) describes that light emitting efficiency outside the device was improved about 1.6 times from 0.51 m/W to 0.81 m/W.
In the earlier Reference (M. Hamaguchi and K. Yoshino, Jpn, J. Appl. Phys. Vol. 34, P. L712, in 1995), detailed measurements were made on the light emitting anisotropy and discharge efficiency of the oriented organic electroluminescence device. According to FIG. 1 thereof, a remarkable difference in the amount of light to be discharged is observed in the direction parallel to the orientation and the direction vertical thereto. By contrast, no marked difference in the amount of the discharged light is observed between the oriented sample and non-oriented sample.
As described above, methods of improving the light emitting efficiency over the previous level are being studied for the organic electroluminescence device. Since many such factors are included, no definite guideline has been established as yet.
To put such an organic electroluminescence device into practical use, it is essential to improve the external quantum efficiency. There is an upper limit to the external quantum efficiency of the above-mentioned conventional organic electroluminescence device. One of the methods is to improve the singlet exciton generation efficiency ηr of the light emitting quantum efficiency of the light emitting material itself, and to improve the external quantum efficiency outside the device by improving the discharge efficiency ηext. Of these, the latter proposal is associated with the improvement of discharge efficiency ηext. It is intended to provide a more extensive efficiency improvement.
Namely, when the discharge efficiency of the conventional organic electroluminescence device was analyzed, isotropic light emission inside the light emitting layer was the basis for logical analysis. Improvement of discharge efficiency was suggested when molecules were oriented uniaxially or in parallel to the light discharge plane. But there was no clear description of the specific degree of orientation, its orientation or the correlation between the related direction of light emission and the structural orientation direction of the molecules. Therefore, the relationship between the molecule orientation direction and orientation direction to provide the optimum discharge efficiency was not necessarily clear. For this reason, it was not possible to perform absolute quantitative design regarding the obtained spatial orientation for light emission.
Furthermore, systematic and concrete study has not been made to clarify the relationship among the polarization and double refraction of the light emitting component itself caused by forming the state of anisotropic light emission, changes in the ease of movement of related electrons and holes, and the state of the boundary between the device and the outside of the device to discharge their light emitting characteristics out of the device in the final phase. In addition, no study has been made on the relationship with various intermediate layers existing between the light emitting layer and device boundary. The impact of these factors upon the emission spectrum distribution has not been studied. For this reason, no sufficient achievement has been made in terms of improvement of the external quantum efficiency based on the discharge efficiency improvement technique of conventional devices.
A control method by a fine resonator having a resonance length on the order of a wavelength is known as a orientation control means for an emission pattern. For example, the Reference (S. Tokito, Y. Taga and T. Tsutsui, Synthetic Metals, Vol 91, P. 49, 1997) includes a report on a fine resonator structure type organic electroluminescence device where tris(8-quinolinolato) aluminum (Alq3) is used as a light emitting material, and MgAg is employed for the electrode cum reflector on the back side, ITO for the electrode on the light discharge side, and (SiO2/TiO2) derivative multi-layered film for the translucent mirror on the light discharge side. FIGS. 3 and 4 show that the directivity of the emission pattern is improved by the introduction of a fine resonator structure. When the light intensity is “1” on the front of the light emitting device, the radiation angle where the light intensity is reduced to a half is about 60° when there is no fine resonance structure. By contrast, the light intensity is reduced to about 20° when a fine resonance structure is used, according to this Reference. However, in this method, the directivity of radiation differs according to the light wavelength, and there is a big change in the spectrum depending on the angle of view. At the same time, the directivity is increased extremely. So when it is used as a display for an organic electroluminescence device, there has been a problem that the angle of the field is reduced.
Another method is disclosed in the Reference (Japanese Patent Laid-Open No. 102783/1999). According to this method, a light emitting layer is formed in vacuum by a dry process, and the organic compound molecule constituting said light emitting layer is oriented in parallel to the light emitting surface, thereby improving the light emitting efficiency. According to this report, the light emitting organic molecule is oriented randomly in three dimensions in the conventional light emitting layer, and a light emitting efficiency of about 0.2 was the limit. According to this technique, the light emitting efficiency is excellent in the direction vertical to the light emitting surface. In this case, the light emitting molecule may be randomly oriented in two dimensions within the surface parallel to the light emitting surface, according to said Reference. In this technique, however, when the light intensity on the front of the light emitting device is “1”, radiation angle where the light intensity is reduced to a half is still about 20°, as shown in FIG. 3, and the directivity is too high. This has been a problem.
Namely, the optimum comprehensive conditions for the device have not been proposed for the overall improvement of the characteristics in an organic electroluminescence device, such as light emitting efficiency, discharge efficiency, directivity and anisotropy.
Furthermore, there has been no proposal on the image display system and configuration method thereof using the effects specified said organic electroluminescence devices which cannot been observed in other flat panel displays, or on the method of use, for example, in video distribution services based on said system which cannot be observed in the conventional video or sound broadcasting services.